The present invention relates generally to seismic hydrocarbon exploration, and more particularly provides an earth impedance determination and compensation system for use with a seismic vibrator or the like.
Seismic exploration for oil and gas typically utilizes various seismic energy sources to excite seismic waves which probe the subsurface geological structure of the earth. The reflected seismic waves return to the earth's surface, are detected by surface geophones or seismometers, and then are recorded. These signals are operated on utilizing digital computers to manipulate the seismic data to produce a processed seismic section, as a function of two-way travel time and the lateral spatial coordinate. The processed seismic section is then analyzed or interpreted to infer the subsurface geology and the structure of the earth. This interpretation is used to identify and delineate structures where hydrocarbons are likely to accumulate.
In its simplest form, the seismic reflection exploration technique is an echo-ranging technique, similar to the radar or sonar system, in which the travel time of the reflected seismic wave is the desired information. Historically, only the time measurements of the reflection events were considered in seismic reflection profiling. This travel time method was useful in exploring and mapping the earth's subsurface structure wherein structural features are likely to trap hydrocarbons as they migrate through the porous zones in the earth. Countless discoveries of oil and gas were made throughout the world by mapping these structural traps. However, this method is an indirect one. Hydrocarbon accumulations are not mapped directly, but structures where hydrocarbons might accumulate are mapped, and these structures are subsequently tested by drilling.
Exploration objectives have changed gradually from the large-scale structural traps as both the number and size of undiscovered structural traps have diminished. The more prevalent objective now is to map the more subtle stratigraphic traps which are more numerous and are believed to contain several times the quantity of hydrocarbons estimated to exist in undiscovered structural traps. As this focus has changed, so has seismic exploration technology. It has advanced and become more sophisticated since mapping stratigraphic variations is a more formidable problem. Instead of reflected seismic wave travel time, other features in the received signals, such as variation in amplitude and phase or wave shape, have become more and more important since they provide information that may make it possible to detect hydrocarbons directly. The amplitude and wave shape of recorded signals depend on many factors. These include, among others: (1) seismic sources, (2) their couplings to the ground, (3) near-surface weathered layers, (4) the nature and characteristics of the receiving and recording instruments, and (5) the reflection characteristics from deep geological structures.
Of these five factors, the reflection characteristics from deep geological structures is the pivotal factor in determining the nature of such deep geological structures. The other four factors tend to "mask" or alter the form of the reflected waves from the deep geological structures of interest and, accordingly, must be controlled or accounted for to provide a true reflective picture of the deep layer structure of interest. This is especially true if it is desired to map the lateral stratigraphic and lithological variation of the deep layers. Once these factors have been in some manner taken into account, the subtle features that are solely determined by the deeper structures with hydrocarbon potential can then be inferred. However, this inference process is presently quite complex for a variety of reasons.
One of the more important direct hydrocarbon detection techniques is the "bright" spot or hydrocarbon indicator (HCI) technique in which the amplitude of reflections are mapped. Reservoirs saturated with gas have different properties than brine-saturated reservoirs. High amplitude reflections are indicative of gas saturization. This technique has been very useful in the marine environment, but has enjoyed considerably less success when applied on land. A key reason for the lack of success on land is that, in the marine environment, both the sources and receivers are in a homogeneous medium (water) and observed lateral variations in reflection amplitude maybe attributed to lateral variations in the reflectors due to pore fluid differences at reservoir depths. On land, the variations in the earth's surface are too great. In land seismic acquisition, the most widely used seismic source is known as the "VIBROSEIS" system, a system developed by the Continental Oil Company in the 1950's. In this system, and other systems like it, a servo-hydraulic vibrator actuates a steel baseplate pressed against the ground. An oscillatory motion rather than an impulsive one is utilized. The seismic signal usually consists of an oscillatory wave form of slowly varying frequency, typically within the frequency band 10 to 100 Hz, and lasts for 7 seconds or longer. The advantage of the vibroseis and similar systems stems from the fact that total energy is spread over a long time. The instantaneous power is, therefore, greatly reduced and is less sensitive to noise bursts. The system can be advantageously utilized even in cities and towns where the use of dynamite or other explosives may be dangerous and prohibited. The need for shotholes for explosives is also avoided.
The seismic wave form generated by the vibroseis system is a long wave train. The received signals, therefore, may be thought of as a superposition of many long wave trains with different amplitudes and time delays produced by reflecting horizons in the subsurface structures. Special processing techniques are required to make recorded signals interpretable. The processing technique amounts basically to looking for the long swept-frequency signal in the recorded wave train, e.g., comparison of these two signals with a progressive increase in time delay. The most meaningful quantitative statistical measure of similarities of signals is the cross-correlation function. The cross-correlation between the recorded signal and the input signal is a way to reduce the recorded signal and to obtain useful information about geology from the vibroseis system. In idealized situations, each reflection event after cross-correlation is represented by a short-duration, zero-phase wavelet (symmetric wavelet). This wavelet is the autocorrelation of the signal sent out by the vibrator. After cross-correlation, the resultant trace is similar to the seismic trace obtained with an explosive source, and further analysis or interpretation may proceed in the same way.
If the exploration objective is the detection of subtle stratigraphic traps, the amplitude and phase of the radiated seismic source must be tightly controlled. In the case of the vibroseis system, the vibrator is connected to a base plate pressed against the ground. At different locations, the ground reacts with different elastic impedances to the motion of the base plate. Therefore, in general, the vibroseis system's seismic radiation will be different at different locations even when the same electrical drive signals to the vibrator are used.
It is well known that if the vibroseis system is operated in an open-loop fashion, the radiated seismic wave does not have a constant-phase relationship with the electric drive signal used as a reference in the cross-correlation process. It is customary, however, to operate the vibroseis system in a closed loop fashion with a phase-lock mechanism to insure the output is phase-locked to the electrical drive signal. To achieve this phase-lock mode, it is customary to utilize an output signal from the vibrator to correct the drive signal to achieve the desired phase-lock. Typically, such vibrator output signal is the acceleration of the base plate, the reaction mass portion of the vibrator, or a weighted average of the two accelerations (with respect to masses). The purpose of these phase-lock techniques is to control the form of the seismic waves generated into the earth so that known generated wave forms can be used in the cross-correlation process. However, because of variation in the earth's impedance at different generating and receiving locations, it has been found that even if phase-lock is achieved, the generated wave form transmitted into the earth varies at different surface locations. It is this heretofore uncompensated for variance in earth impedance which has rendered the cross-correlation process inaccurate and difficult, concomitantly rendering the use of the "bright" spot seismic exploration method difficult and inaccurate in land applications.
Recent advances in technology regarding the control of the vibrator have been directed at the problem of base plate decoupling from the ground. This situation can occur at high-drive levels using the base plate acceleration as a reference. When such decoupling occurs, harmonics of the control signal are generated, and there is an overall degradation in the quality of the process seismic data. As exemplified in U.S. Pat. No. 4,184,144, a method known as "ground-force control" has been developed to minimize the base plate's decoupling by controlling the force of the base plate on the earth. In this ground force control method, a control signal is used which is a weighted average of the base plate acceleration signal and the reaction mass acceleration signal. By using this weighted average of such vibrator output signals, the ground force of the base plate may be controlled to prevent base plate decoupling. However, implementation of this method still does not assure that a seismic wave of predetermined form is actually being transmitted into the earth. This important deficiency is again due to failure to account for the variation in the earth's impedance at various surface locations.
This variation in earth impedance, which can cause the cross-correlation process to be inaccurate and difficult, and has heretofore rendered the "bright" spot exploration technique rather ineffective in land applications, is due primarily to impedance variations in the near-surface "weathered" layer of the earth. Furthermore, this variation in earth impedance requires using a statistical rather than a deterministic static correction process, which is more prone to errors. The weathered layer is the layer at the surface of the earth that has very low seismic velocity and density. It might have rapid spatial variation in thickness that may even be subject to seasonal fluctuation in physical properties due to rainfall and other weather conditions. The reaction impedance of this layer to the vibrator force, as previously mentioned, will be different at different locations. Thus, even when the force of the vibrator can be made to be identically the same (e.g., when the "ground-force control" system is utilized) there is simply no guarantee that the amplitude and phase of the generated seismic wave will be the same as it passes through the weathered layer of the earth as different locations thereon. Accordingly, this same uncertainty has existed with respect to the form of seismic waves actually being transmitted into the sub-weathered layer portions of the earth that are being probed with the seismic reflection profiling method.
The effect of the weathered layer is not just a constant time shift or a constant amplitude factor which can be accounted for by the usual static and amplitude corrections used in seismic processing. The contrast between the weathered layer and the sub-weathered layer is usually quite large. Therefore, the weathered layer will cause severe reverberation problems. Depending on the travel time in the weathered layer, the effect may be separable as different reflections or may be superposed with the overall change of the wave shape in the cross-correlated record section. In either event, such effect will contaminate the reflection events from the deeper geological structures which may contain hydrocarbons. The presence of the weathered layer will change the amplitudes and the phases of the seismic waves that propagate through it. The effect is highly frequency-dependent and also changes the radiation patterns of the vibrator.
Another impedance-related problem has been that because of impedance variances it has been difficult to accurately determine the compressional and shear wave forms (the "P" and "S" wave forms) of the seismic waves being propagated into the earth in order to properly arrange an array of seismic power sources to optimize the generation of "useful" seismic waves and attenuate undesired seismic waves. Heretofore such array arrangement has been conducted on a laborious trial and error basis founded upon often inaccurate estimates of such "P" and "S" wave forms or, if done theoretically, has used unrealistic models for the vibrator radiation.
It can be seen from the foregoing that uncompensated for variations in the earth's near surface weathered layer impedance have heretofore presented a barrier to the most effective implementation of the seismic exploration technique to land applications and, in particular, has limited the effectiveness of the "bright" spot technique. It is accordingly an object of the present invention to provide apparatus and methods for use with seismic vibrators, and other seismic energy sources, which will determine and compensate for variation in earth impedance in a manner eliminating or substantially minimizing above-mentioned and other problems and limitations associated with conventional seismic land exploration systems.